This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-399217 filed Dec. 27, 2000, the entire contents of which are incorporated by reference.
1. Field of the Invention
The present invention relates to an oscillation circuit, and more particularly, to an oscillation circuit of a cascode connection type. Further, the invention relates to a technique for preventing a phenomenon in which the oscillation frequency of an oscillation circuit varies due to changes in power supply voltage.
2. Description of the Related Art
To construct an oscillation circuit for generating a high-frequency oscillation signal, a cascode connection is being widely used. The cascode connection is a connection method for connecting a grounded-emitter transistor circuit to a grounded-base transistor circuit in series. The cascode connection is characterized in that it can provide an oscillation circuit with both a total gain equivalent to that of the grounded-emitter transistor circuit and a bandwidth up to the cutoff frequency of the grounded-base transistor circuit. Accordingly, the cascode connection is very useful in constructing a high-frequency oscillation circuit.
FIG. 1 is a circuit diagram illustrating a conventional grounded-collector oscillation circuit 10 of a cascode connection type. As shown, the oscillation circuit 10 comprises two bipolar transistors 11 and 12, bias resistance elements 13-15, a resistance element 16, a load 17, a capacitance element 18 and an LC oscillator section 19.
The transistor 11 includes a base connected to respective ends of the bias resistance elements 13 and 14, a collector connected to an end of the load 17, and an emitter connected to the collector of the transistor 12. The transistor 12 includes a base connected to the other end of the bias resistance element 14, an end of the bias resistor 15 and the output terminal of the LC oscillator section 19, a collector connected to one electrode of the capacitance element 18, and an emitter connected to an end of the resistance element 16. The other ends of the bias resistance element 13 and the load 17 are connected to a power supply potential Vcc. The other ends of the bias resistance element 15 and the resistance element 16, and the other electrode of the capacitance element 18 are connected to the ground potential. In other words, the grounded-base transistor 11 and the grounded-emitter transistor 12 are connected in series. A capacitance element 20 interposed between the base and collector of the transistor 12 is a parasitic capacitance element. The capacitance element 20 is created because of a depletion layer which occurs in a pn-junction between the base region and collector region of the transistor 12. Therefore, the capacitance Cbc of the capacitance element 20 depends upon the width of the depletion layer. The width of the depletion layer depends upon the base-collector voltage Vbc of the transistor 12.
In the oscillation circuit constructed as above, the difference in potential between the bases of the transistors 11 and 12 (a voltage drop occurring at the bias resistance element 14) is (R2xc2x7Vcc)/(R1+R2+R3) (R1xc2x7R3: the respective resistances of the bias resistance elements 13-15). Thus, the potential difference is directly determined from the resistances of the bias resistance elements 13-15. Accordingly, the base-collector voltage Vbc of the transistor 12 is ((R2xc2x7Vcc)/(R1+R2+R3)xe2x88x92Vbe). Vbe represents the base-emitter voltage of the transistor 11, which is substantially constant. As is evident from the above equation, a change in the power supply potential Vcc causes a change in the base-collector voltage Vbc of the transistor 12.
Moreover, as aforementioned, the depletion layer width that determines the capacitance Cbc of the capacitance element 20 depends upon the base-collector voltage Vbc of the transistor 12. Further, the base-collector voltage Vbc depends upon the power supply potential Vcc. Consequently, the capacitance Cbc depends upon the power supply potential Vcc, which means that a change in the power supply potential Vcc causes a change in the capacitance Cbc.
The oscillation frequency fosc of an oscillation signal output from the oscillation circuit 10 is basically determined from an oscillation signal output by the LC oscillator section 19. However, the oscillation frequency is, of course, influenced by the capacitance Cbc of the capacitance element 20. This being so, a change in the power supply potential Vcc causes a change in the base-collector voltage Vbc of the transistor 12, which causes a change in the capacitance Cbc of the capacitance element 20 and hence a change in the oscillation frequency fosc of the oscillation circuit 10.
As described above, a phenomenon in which power supply potential Vcc fluctuation causes oscillation frequency fosc fluctuation (this phenomenon is called xe2x80x9cpushingxe2x80x9d) occurs in the conventional cascode-connection-type oscillation circuit.
An oscillation circuit according to an aspect of the present invention comprises:
a first transistor including a base inputted an oscillation signal, an emitter connected to a ground potential, and a collector;
a second transistor including a collector connected to a power supply potential, a base and an emitter; and
a first resistance element having one end connected to the collector of the first transistor, and another end connected to the emitter of the second transistor, the first resistance element causing a voltage drop proportional to the power supply potential,
wherein the a resistance of the first resistance element is set such that a change in a potential appearing across the first resistance element is equal to a change in a potential difference between the bases of the first and second transistors when the power supply potential changes.